Vehicle head restraints typically include an upper cushion and a pair of parallel support posts that are inserted down into a guide member within the top of the seat back frame, and moved up or down to a desired adjusted height above the top of the seat back. While powered height adjustment systems exist, the much more common system is manual, in which the cushion is simply pulled or pushed up or down by the seat occupant. The support posts are generally held in place by a friction mechanism at any position between a lowest and highest limit position, or at any one of series of discrete positions.
Until relatively recently, a head restraint was static, apart from the up and down height adjusting motion. A recent coassigned U.S. Pat. No. 5,378,043, disclosed a dynamic design that moved the head restraint forward and up in response to a sudden vehicle acceleration, such as that caused by a rear impact. The head restraint support posts slide within a support, and a target structure plate is pivoted to the horizontal top bar of the seat back frame so as to be able to swing backwardly. This backward swinging motion occurs in response to the backward force of a relatively decelerating seat occupant into the seat back. The backward swinging of the impact target below the seat back frame top bar is matched by a forward swinging of the head restraint cushion and its guide posts above the top bar. More than a simple, rocking forward is involved, however. The guide tubes within which the support posts adjustably slide are designed to slide upwardly relative to the top bar, due to a camming action, which raises the head restraint cushion relative to the top bar. However, how far above the seat back frame top bar the head restraint cushion ultimately raises depends on how far upwardly the head rest cushion and its support posts were originally located. The head restraint does not achieve a consistent final raised position above the seat back frame top bar.